A conventional semiconductor device is positioned on a device formation region of a semiconductor substrate, and is separated by a device separation region thereof. There are two methods for fabricating a device formation region, that is, one is a selective (preferential) oxidation method and the other is a trench separation method.
According to the selective oxidation method, the device formation region is formed by photoetching using a reticle as an etching mask. A conventional reticle is composed of a light transmission layer and a light shield region formed to be a rectangle on the light transmission layer.
In fabrication of a conventional semiconductor device, a first resist pattern is formed on a silicon substrate by photoetching using the reticle as an etching mask. At this time, the first resist pattern is formed to be along a 27% uniform intensity line of light passing through the light transmission layer. Thus, the first resist pattern has two curved portions each having the same radius.
Next, a field oxide layer is formed on the silicon substrate as an insulation layer for a device separation region, and a device formation region is formed by etching using the first resist pattern as an etching mask. At this time, the device formation region is shaped to be like a concave mirror being encircled with an inner edge of the field oxide layer.
Next, a polysilicon layer is formed on the whole process surface of the substrate, and a resist layer is formed on a surface of the polysilicon. After that, the resist layer is etched, so that a second resist pattern for gate electrodes is formed thereon.
Finally, the polysilicon layer is etched with using the second resist pattern as an etching mask, so that the gate electrodes are formed on the device formation region.
According to the conventional semiconductor device, however, there is a disadvantage in that, when the device formation region is formed, radiation light is converged to an inside of the device formation region by light reflection on the inner edge of the field oxide layer, because the device formation region is shaped to be like a concave mirror. Then, the reflected light is radiated to a side surface of the second resist pattern, so that the side surface which is an inaccurate portion is also etched. Therefore, depressions are formed at the second resist pattern, so that the gate electrodes are provided with depressions corresponding to the depressions of the second resist pattern. As a result, a reliability of the semiconductor device is lowered.